1. Field of the Invention
The invention relates to a test system for dynamically and/or cyclically load testing a material sample, a component or an assembly, referred to in the following as the sample. The test system comprises a test frame to which a bearing and a counter bearing are attached. At least one actuator having a multifunctional solid state transducer material system is connected to the counter bearing, directly or indirectly, to which actuator, also directly or indirectly, a fastening means is attached for one-sided detachable, firm fixation of the sample. Likewise on the bearing side, a fastening means for one-sided detachable, firm fixation of the sample is provided such that the at least one actuator is able to introduce dynamic and/or cyclical mechanical loads into the sample, which act along a load path extending lengthwise between the bearing and the counter bearing and through the sample.
2. Description of the Prior Art
For the experimental investigation of the mechanical dynamic load bearing capability of material samples, components or assemblies having multiple components, test systems of the type described above are used to apply compressive and/or tensile loads to a sample under investigation in a dynamic, cyclical sequence. During the cyclical load application, the sample undergoes a continuous fatiguing process, which after numerous cyclical stress applications, that is load reversals, results in material degradations and associated crack formation with accompanying failure of the material.
In general, all cyclically loaded samples have a limited number of reversals and the amplitude of stress that acts cyclically on the sample. The capacity of a sample to withstand stress and the service life of the sample may be qualified on the basis of a number of load reversals the sample is able to sustain before macroscopically detectable signs of fatigue occur. The following four load reversal ranges have become established with regard to qualifying the service life of samples: The term low-cycle fatigue (LCF) is used when a sample fails after about 103 load reversals. A sample has high cycle fatigue (HCF) if signs of fatigue begin to appear in the sample after about 108 load reversals. The next load reversal frequency category is defined for samples that have a “high durability”, (very high fatigue=VHF) and are able to sustain up to 1010 load reversals without damage. However, there is still some disagreement whether in general true durability is indicated, or whether failure occurs also even with very low stress amplitudes in the case of very load reversal numbers. Materials are considered to fall in the category of “ultra-high cycle fatigue” if they are able to sustain more than 1010 load reversals without damage.
Performing dynamically cyclical sample fatigue tests in which samples are to be exposed to 106 or more load reversals is constrained by the use of conventional testing techniques, such as those that rely on servohydraulic test actuators which is financially impractical, particularly with respect to temporally significant limits. It is therefore obvious that in order to perform extremely large numbers of load reversals in the order of 1010 to 1012, the sample under investigation must be subjected to a mechanical vibration excitation with frequencies of 1 kHz and higher in order to make it possible to detect changes in fatigue strength for such an extremely large number of load reversals during a relatively short experiment period. It is precisely this object that was pursued in a method described in German application DE 10 2007 038 479 A1 for assessing the fatigue behavior of a material in which a sample to be tested is caused to vibrate at frequencies≧5 kHz by cyclical mechanical excitation.
An actuator having a piezoelectric material, preferably in the form of a piezoelectric stack actuator such as is used to generate mechanical stresses in a sample in a material testing machine in document PS 29 39 923 C2 is suitable for generating stress vibrations with frequencies of a few kHz that act in targeted manner on a sample. The test frame is constructed as a column-type test stand and comprises a counterweight supported on a foundation via elastic elements, on which the test frame, having a vertical strut and an upper crossbeam, is braced. The lower end of a material sample to be examined is clamped on one side directly to the lower counterweight via a counter bearing. The upper end of the material sample is enclosed by a bearing mounted on a vibration mass that is permanently attached to a piezoelectric stack actuator via an oscillating spring. The actuator is rigidly mounted on the crossbeam of the test frame. Since piezoelectric actuators are only able to produce small travel paths for vibrational excitation, the excitation frequency of the stack actuator is selected such that it matches the natural frequency of the vibrating spring-mass system of the material sample, the vibration mass, the oscillating spring and the test frame, so that the low excitation forces originating from the piezoelectric actuator are able to generate large inertia forces, which are taken up accordingly by the sample. Besides the “resonance mode”, the known material test machine enables the sample to be examined in a “follow-on mode”, that is, the material sample may be tested under desired stresses relative to forces with any predetermined test frequencies by a direct connection to the piezoelectric stack actuator via suitable connecting elements of the sample.
It is true that since its first use, a sensor system in a state of continuous technological evolution for detecting material degradations caused by the progressive material fatigue during the load reversal test has enabled physically detectable measurement parameters, for example oscillation amplitude, number of vibrations, acceleration forces acting on the sample, degradation effects on the sample surface, such as crack formation, etc. to be measured with ever increasing accuracy. But at the same time equally undesirable disturbances are also measured in the same way with equal accuracy, and these distort the measurement results.
Consequently, the conclusion is reached that cyclical stress tests of samples using conventional test machines with typically designed test frames, in the manner of a column test stand at test frequencies of >100 Hz are prone to significant error effects. As a result the quality of the overall test result may at least be cast into doubt. Particularly when testing modern samples and materials, which are intended, for example, to enable reliable lightweight construction for long-term and continuous use, it is essential to use reliable, highly dynamic testing techniques. Such test techniques, with which at least 108 load reversal cycles are to be achieved, require test frequencies of at least several hundred Hertz, preferably 1 kHz and more. In particular, it is important to ensure that at such high test frequencies no critical vibration excitations are permitted to occur inside or outside the test device. Thus, it is important to prevent any undesirable resonant oscillations in the test system, in the sample being tested, and in structures close to the test system. It is also imperative to completely decouple the test system from its environment in a vibration-relevant manner.
An apparatus for conducting fatigue tests on a sample is disclosed in U.S. Published Application 2002/0017144 A1, in which a sample clamped at both ends is subjected to slow tensile or compressive alternating loads in the lengthwise direction, which are generated via an electromechanical spindle drive, a motor or a servohydraulic drive unit. At the same time, the sample is subjected to rapid alternating bending stresses provided by a piezoelectric actuator that acts on a fastening means and in a direction transversely to the tensile or compressive alternating loads. Excitation takes place at the resonance frequency of the sample until cracks appear and the sample breaks. U.S. Pat. No. 6,023,980 discloses a fatigue test device for high load cycle numbers with which vibration frequencies between 1 and 4 kHz can be produced, corresponding to typical vibration frequencies to which for example turbine blades are exposed. The apparatus comprises an inner frame, having two plates, and four rods that connect the plates. The sample is subjected to a static load, and to a dynamic load by two piezoelectric actuators with one acting on each of the samples.
Patent No. WO 2011/086254 A1 (D3) describes a device for monitoring mechanical properties of a viscoelastic material, in this case particularly a solid rocket fuel, with regard to any aging that manifests itself as softening of the material, due to moisture, for example, or as hardening, due to crosslinking of the material, for example. For this purpose, a sample of the material is brought into contact with a vibration source via a plate. Vibration signals are captured by a sensor at the opposite end of the sample.